1. Field of the Invention
This invention relates generally to a piezoresistive device employing boron nitride as the piezoresistive material and, more particularly, to a piezoresistive pressure sensor employing boron nitride as the piezoresistive material for use in hostile environments, and a piezoresistive microbolometer employing boron nitride as the piezoresistive material for use in focal plane arrays and the like.
2. Discussion of the Related Art
The automotive industry needs a more stable, high temperature pressure sensor for sensing pressure in cylinder heads, brake fluid systems and other hostile environments. Currently available commercial pressure sensors cannot adequately operate in these types of automotive environments because their performance is unstable in such environments. For example, conventional pressure sensors are not applicable to sense the pressure within a vehicle cylinder and in vehicle brake lines because of the high heat. In order to meet increasing legislative and design demands, new sensors must be developed to monitor these types of environments.
One known pressure sensor is a micro-machined pressure sensor formed by an integrated processing technique employing a silicon substrate, where oxide isolated piezoresistors are mounted directly to a silicon diaphragm. Silicon piezoresistors have a gage factor that can range between 50 and 200, which is a significant improvement over metals. These sensors have been adequate for sensing pressure in non-hostile environments, but in high temperature and high impulse environments, the high leakage currents and brittle silicon diaphragm are generally not adequate. The best known sensor for use in a hostile environment has been silicon piezoresistors grown on sapphire bonded to a titanium substrate. However, these sensors can have serious drift problems due to annealed stresses, which can cause zero drift when in use, thus limiting their ability to be effective in hostile environments.
Piezoresistivity is the property of a material that results in the observed change in the resistance of the material under the influence of an applied stress. To a first approximation, piezoresistivity can be viewed as a primarily geometric effect resulting from the applied stress. Consider a fine wire of uniform cross-section. If xcfx81 is the resistivity (xcexa9m), l is the length (m), and a is the area of the cross-section (m2), the resistance R=xcfx81l/a(xcexa9).
If a uniform stress ("sgr"(Nmxe2x88x922)) is applied along the length of the wire, then:
dR/d"sgr"=d(xcfx81l/a)/d"sgr"=xcfx81/axc2x7∂l/∂"sgr"xe2x88x92xcfx81l/a2xc2x7∂a/∂"sgr"+l/axc2x7∂xcfx81/∂"sgr"xe2x80x83xe2x80x83(1)
If the change in the resistance is then compared to the initial value of resistance, the result is:
xe2x80x83dR/R=∂l/lxe2x88x92∂a/a+∂xcfx81/xcfx81xe2x80x83xe2x80x83(2)
For a circular wire,
a=xcfx80xc2x7d2/4 and xe2x88x92∂a/a=xe2x88x922∂d/dxe2x80x83xe2x80x83(3)
Since the change in diameter (d) of the wire is related to the change in length (l) of the wire by Poisson""s Law, then:
v=∂d/d/∂l/lxe2x80x83xe2x80x83(4)
Then equation (2) can be rewritten as:
dR/R=∂l/l(1+2v)+∂xcfx81/xcfx81xe2x80x83xe2x80x83(5)
For metals, the change in the resistivity (xcfx81) can be related to the change in the volume (V) through the Bridgeman Constant (C) as follows:
∂xcfx81/xcfx81=Cxc2x7∂V/V and ∂V/V=∂l/l(1xe2x88x922v)xe2x80x83xe2x80x83(6)
Combining equations (5) and (6) gives:
dR/R=∂l/l{(1+2v)+C(1xe2x88x922v)}=G∂l/lxe2x80x83xe2x80x83(7)
where G is the gage factor. Typical values for v and C in metals are v≈0.3 and C≈1.13 to 1.15. This yields a value of G for metallic strain gages of 2.0-2.3.
In the case of semiconducting materials (Si, Ge, BN), the final term (∂xcfx81/xcfx81) in equation (5) dominates. Because the underlying crystalline structure determines the directional sensitivity of the conduction process in a semiconductor, the resistivity changes from a scalar to a tensor. Experimentally, the nine coefficients that are normally found in such a tensor have thus far been found to reduce to six and also to form a symmetric tensor. With an applied electric field, the relationship is as shown below.                               [                                                                      E                  1                                                                                                      E                  2                                                                                                      E                  3                                                              ]                =                              [                                                                                                      ρ                      1                                        ⁢                                          ρ                      6                                        ⁢                                          ρ                      5                                                                                                                                                              ρ                      6                                        ⁢                                          ρ                      2                                        ⁢                                          ρ                      4                                                                                                                                                              ρ                      5                                        ⁢                                          ρ                      4                                        ⁢                                          ρ                      3                                                                                            ]                    ⁡                      [                                                                                i                    1                                                                                                                    i                    2                                                                                                                    i                    3                                                                        ]                                              (        8        )            
Boron niitride (BN) has become an important material in the electronics industry because it is a wide band gap semiconductor material with high thermal conductivity and chemical inertness. Semiconducting thin film boron nitride has been developed for application as an electron emitting cold cathode material for use in vacuum displays and the like. U.S. Pat. No. 5,646,474 issued to Pryor Jul. 8, 1997 discloses a cold cathode of this type. Boron nitride exists in several crystalline structures, and may be amorphous, polycrystalline or a single crystal when used in the cathode emitter.
It has been suggested by the present invention that boron nitride be used as the piezoresistive material in a pressure sensor for use in hostile environments. It is an object of the present invention to provide such a sensor.
In accordance with the teachings of the present invention, a piezoresistive device is disclosed that makes use of boron nitride as the piezoresistive material. In one embodiment, the device is a pressure sensor where the boron nitride enables the sensor to provide suitable performance in hostile environments. The sensor can include a titanium substrate covered with a diamond insulator layer. An n-type boron nitride piezoresistive element is deposited on the diamond layer and is electrically connected to electrical contacts. The sensor is then electrically connected to a suitable sensor circuit.
In an alternate embodiment, the device can be a microbolometer. A cantilevered substrate including a boron nitride piezoresistive element configured within a suitable dielectric extends over a well. The differences in the coefficience of thermal expansion between the dielectric and the piezoresistive element causes the dielectric to curl in response to increased temperature. This curling of the substrate causes the resistance of the boron nitride element to change, which can be measured and give an indication of the temperature.
Additional objects, advantages and features of the present invention will become apparent from the following description and the appended claims when taken in conjunction with the accompanying drawings.